The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the reduction of scan time when imaging two or more anatomic regions of a subject.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M1. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The present invention will be described in detail with reference to the well known Fourier transform (FT) imaging technique, which is frequently referred to as “spin-warp”. The spin-warp technique is discussed in an article entitled “Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging” by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (ΔGy) in the series of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed. The number of views that must be acquired for a complete NMR data set is determined by the size of the field of view (“FOV”) along the phase encoding gradient axis direction and the prescribed spatial resolution of the desired image along that axis. For a given image resolution, the larger the FOV along the phase encoding axis the larger the number of views that must be acquired and the longer the resulting scan time.
When the anatomy of interest is smaller than the anatomy inside the bore of the MRI system, one can reduce scan time for a given resolution by reducing the FOV to encompass only the anatomy of interest. One difficulty with this strategy is the phenomenon of phase wrap-around or “aliasing” in which anatomy located outside the FOV along the phase encoding axis produces NMR signals that superimpose artifacts into the FOV due to the image reconstruction process. These superimposed image artifacts complicate the image and reduce its clinical value. There are artifact suppression techniques such as “presaturation” for dealing with this problem, but these methods work by suppressing all signals from outside the FOV.
There are a number of clinical applications in which images are acquired from two distinct regions of the anatomy. For example, it is common practice to acquire an MR image from both legs of a subject during a single scan. Such methods typically employ a local receiver coil for each leg, but because of the aliasing problem discussed above, a single, large FOV that includes both legs is prescribed. Aliasing artifacts caused by the other leg cannot be suppressed using presaturation in this situation because the other leg is also being imaged at the same time. As a result, the prescribed FOV must be much larger than the anatomy of interest in order to avoid aliasing artifacts. This substantially increases total scan time because more views are required for the larger FOV.
The use of separate coils for imaging separate fields of view has been studied previously J. S. Hyde, et al., J. Magn. Respectively. 70, 512-517 (1986), Y. Li et al., Anal. Chem. 71, 4815-4820 (1999). However, in these works the efficiency improvement was due to multiplexing the data from multiple coils into a single receiver channel. More recently SENSE, K. P. Pruessman, et al., Magn. Reson. Med. 42, 952-962 (1999), methods allow reduced FOV and reduced acquisition time using known, measured, non-zero coil sensitivity functions.